Participation in regular physical activity has been shown to provide considerable mental and physical health benefits, including the prevention and treatment of depression and the reduction of an individual's risk of overall mortality, cardiovascular disease, and cancer. However, less than 50 percent of U.S. adults engage in physical activity consistent with the surgeon general's recommendations, and nearly 16 percent are completely sedentary. Exercise interventions have limited long-term success, such that half of individuals starting a new exercise program quit within 6 months. The inability of theoretical perspectives on exercise behavior to reliably predict behavior from intentions to exercise has been termed the "intention-behavior gap", a phenomenon that likely is manifest in lack of long-term maintenance of exercise. Evidence that exercise serves as effective mono therapy and adjunctive therapy for depression suggests that long-term maintenance of an exercise regimen may aid in not only the primary prevention of depression, but also in secondary prevention and reduced risk of relapse. Integrating key constructs from the Theory of Planned Behavior and regulatory processes and types of motivation described by Self-Determination Theory, this application proposes a theoretical model with the potential to better predict exercise maintenance and to help explain the intention-behavior gap. The degree to which these proposed mechanisms apply to both clinical and non-clinical populations may aid in the development of more effective exercise interventions. In the current research, an integrated model of exercise maintenance will be proposed and tested (Specific Aim #1). Specific Aims #2 and #3 involve an exploration of the extent to which daily success or failure with unique attempts at exercise influences the temporal stability of model constructs, and testing the ability of temporal stability to predict long-term maintenance via a better explanation of the intention- behavior gap (Specific Aims #2 and #3). Specific Aim #4 considers depressive symptoms and depression as potential moderators of these effects. Study 1 will investigate these aims in a college freshman population, a group at risk for decreased physical activity and weight gain. Study 2 (Specific Aim #5) will use a community sample of adults to explore the external validity of the model. This application is relevant to the interests of public health because of its implications for increased physical activity in sedentary and inconsistently active populations, including those at risk for developing poor exercise habits at an early age (college students) and those at risk for health disparities (individuals in community health settings). Ultimately, this work has the potential for reducing risk of multiple poor health outcomes, including depression and heart disease.